May 3, 2000.
The Geology of the Galapagos Islands.
This is Roger Payne aboard the Odyssey, speaking to you from the Galapagos Islands.
The Galapagos Archipelago straddles the equator in the eastern Pacific. Cocos Island, the nearest land, is about 720 km (that's about 450) miles to the northeast. The nearest point on the mainland is about 960 km (that's 600 miles) away, on the west coast of South America. The archipelago is part of Ecuador and consists of 13 Islands that are greater that 10 square kilometers, plus 6 smaller islands, and over forty islets big enough to have official names. The total land area of all these is about 8,000 square kilometers. The islands are the tips of huge submarine volcanoes. The oldest in the group, the eastern islands, are now extinct. They started forming about 10 million years ago. The western-most islands include active volcanoes. This is because the archipelago sits on a tectonic plate (called the Nazca Plate) that is moving towards South America at a rate of about 7 cm (2 3/4 inches) a year. This may not seem very fast but in the 5 million years since the oldest, still extant island, (older ones have sunken beneath the sea) rose above the surface of the sea that volcano has ridden the plate about 350 km (over 200 miles). The eastern edge of the plate is subducting beneath the South American continental plate (i.e., sliding in underneath it), an activity that builds mountains on the land under which the plate slides-in this case the Andes. It was as the Nazca plate moved over a hot spot in the Pacific that the hot-spot generated a series of volcanic eruptions that built the islands. (A hot spot is a place where magma lies closer to the earth's crust, in this case the sea floor. It gets hotter and cooler in cycles taking thousands of years. When it's hot it erupts through the overlying crust forming volcanoes). Successive eruptions slowly build an island which finally rises above the surface of the sea. The sea then erodes the island and it slowly sinks beneath the waves as the crust cools further making it denser, In the Galapagos, this process is still going on today, and will probably continue for millions more years. In fact the islands are one of the most active oceanic volcanic regions on earth. Since they were discovered, there have been about 6 major eruptions recorded from eight Galapagos volcanoes. Some have been violent but most fairly mild. Here's an eyewitness account about the most violent. It occurred on a February night in 1825, 9 years before Darwin arrived:
"Our ears were suddenly assailed by a sound that could only be equalled by ten thousand thunders bursting upon the air at once; while, at the same instant, the whole hemisphere was lighted up with a horrid glare that might have appalled the stoutest heart. I soon ascertained that one of the volcanoes of Fernandina Island, which had quietly slept for the last ten years, had suddenly broken forth with accumulated vengeance... the heavens appeared to be one blaze of fire intermingled with millions of falling stars and meteors; while the flames shot upward from the peak of Fernandina to the height of at least two thousand feet in the air."
So he raises the sails on his boat and sails away adding:
[...Our boat] slid through the almost boiling ocean at the rate of about seven miles an hour. On passing the currents of melted lave I became apprehensive that I should lose some of my men, as the influence of heat was so great that several of them were incapable of standing. At the time the mercury in the thermometer was at 147 degrees but on immersing it in water it instantly rose to 150 degrees. Had the wind deserted us here, the consequences must have been horrible." Tonight I sailed through precisely the same place he is describing, wondering as we went whether history would repeat itself. With us is a man who climbed to the crater of this volcano and while there survived a violent eruption. Next month I'll get his first hand account of that appalling experience.
Well that's it for tonight. I'll have more to say about the geology of this place tomorrow night.
© 2000 - Roger Payne